In order to improve the deterioration of communication quality due to drastic increases in mobile traffic in recent years and achieve faster communication, the standardization of Carrier Aggregation (CA) functions that enable a radio terminal (User Equipment (UE)) to communicate with a radio base station (eNode B (eNB)) by using a plurality of cells has been undertaken in the 3GPP Long Term Evolution (LTE). Note that the cells that a UE (User Equipment) can use in CA are limited to a plurality of cells of one eNB (i.e., a plurality of cells served by one eNB).
The cells that are used by a UE in CA are categorized into a Primary Cell (PCell) that has already been used as a serving cell when the CA is started and a Secondary Cell(s) (SCell(s)) that is used in addition to the PCell or in dependence thereon. Each SCell can be used by a UE as the need arises, and the use of them can be stopped. Note that starting the use of an SCell is called “activating” or “activation”. Similarly, stopping the use of an SCell is called “deactivating” or “deactivation”. Non-Access Stratum (NAS) mobility information, security information (security input) and the like are transmitted and received through a PCell during radio connection (re)-establishment (RRC connection Establishment/Re-establishment) (see Non-patent Literature 1). A downlink (DL) Carrier and an uplink (UL) Carrier corresponding to a PCell are called “DL Primary Component Carrier (PCC)” and “UL PCC”, respectively. Similarly, a DL Carrier and a UL Carrier corresponding to a SCell are called “DL Secondary Component Carrier (SCC)” and “UL SCC”, respectively.
An operation that is performed to change a radio resource configuration (i.e., RRC information) in an SCell when downlink data (DL data) is transmitted/received in CA is explained with reference to FIG. 11 (Non-patent Literature 2). Here, it is assumed that a UE uses a first cell (Cell1) and a second cell (Cell2) served by an eNB as a PCell and an SCell, respectively. In a step S1, the eNB transmits, to the UE, configuration information for the SCell (i.e., the Cell2) (RRC Connection Reconfiguration (SCell configuration)). In a step S2, the UE performs configuration of radio parameters and the like for the SCell (i.e., the Cell2) (SCell configuration). In a step S3, the UE reports the completion of the configuration of the SCell to the eNB (RRC Connection Reconfiguration Complete).
In a step S4, the eNB transmits to the UE an instruction about the start of the use (called “Activation”) of the SCell (Activation control element (activation of SCell)). In a step S5, the UE starts to use the SCell (SCell activation). In steps S6 and S7, the eNB transmits DL data to the UE by using the PCell or the SCell, or both of them.
In a step S8, the eNB determines that it needs to change the radio resource configuration of the SCell (i.e., the Cell2) and hence transmits to the UE the configuration information related to the SCell that should be changed (RRC Connection Reconfiguration (SCell modification)). In a step S9, the UE updates the configuration of the SCell (SCell modification). In a step S10, the UE reports, to the eNB, the completion of the configuration change of the SCell (RRC Connection Reconfiguration Complete). Note that the eNB does not transmit DL data to the UE in the SCell (i.e., the Cell2) until the eNB receives the report of configuration change completion from the UE.
In steps S11 and S12, the eNB transmits DL data to the UE by using the PCell or the SCell, or both of them.
A radio resource configuration of the SCell in the CA can be appropriately changed through the procedure shown in FIG. 11. Note that data transmission/reception is suspended while the eNB and the UE is configuring or updating radio parameters and the like by the RRC Connection Reconfiguration. For example, the eNB suspends DL data transmission to the UE and the UE suspends the DL data reception from the eNB.